The exemplary embodiment relates to free radical polymerization. It finds particular application in the synthesis of arborescent polymers and copolymers via RAFT polymerization and compositions comprising them. The polymer finds application in synthetic rubber compositions. However, it is to be appreciated that the present exemplary embodiment is also amendable to other like applications.
Arborescent (randomly branched) polymers have a cascade-type structure in which the polymer chains are branched. Such polymers have been synthesized using an anionic grafting method (See, for example, Gauthier, M., et al., Macromolecules, 24, 4548-4553, 1991). This method involves the separate synthesis of narrow molecular weight distribution polymers by anionic polymerization, followed by several subsequent grafting reactions to yield higher degrees of branching. The method does not lend itself to industrial processing due to stringent conditions and separate subsequent reaction steps used to build the arborescent polymer. The resulting arborescent or polymers also have a narrow molecular weight distribution which does not necessarily yield optimum physical properties.
Arborescent polyisobutylene (arb-PIB) of high molecular weight has been synthesized by inimer (initiator-monomer)-type living carbocationic polymerization of 4-(2-methoxyisopropyl) styrene and 4-(1,2-epoxisopropyl) styrene inimers. Subsequent development and study of these materials has resulted in the creation of arborescent block copolymer thermoplastic elastomers (TPEs) such as arb-PIB-b-PSt, arb-PIB-b-PpMeSt and others. These TPEs have shown to have a superior combination of properties compared to their linear tri-block counterparts. Additionally, these materials can be readily formed by one-pot synthesis.
Rizzardo, et al. report attempts to use vinylbenzyl dithiobenzoate (a mixture of meta and para isomers) in the synthesis of poly(methylmethacrylate-graft-styrene) (WO98/01478). The vinylbenzyl dithiobenzoate was co-polymerized with methylmethacrylate using azobisisobutyronitrile (AIBN) as the free-radical initiator at 60° C. The resulting poly(vinylbenzyl dithiobenzoate-co-methylmethacrylate) was used as a chain transfer agent in mediating the bulk polymerization of styrene. However, the authors report that the reaction resulted in a gel material.
Reversible addition-fragmentation chain transfer (RAFT) is a type of controlled free-radical polymerization (CFRP). The mechanism of RAFT is understood to rely on degenerative chain transfer through the dithioester functionality of the chain transfer agent (CTA) to produce polymers of narrow molecular weight distribution (MWD) and is thus a versatile type of CFRP. This mechanism is applicable to a wide range of monomers, solvents, and temperatures. Polymerizations have successfully been carried out in bulk, solution, emulsion or suspension to produce linear, graft, block, and star (co)polymers. However, there are several disadvantages with RAFT polymerization. The dithioester chain transfer agent has a pungent odor, which makes it undesirable to work with, and the resulting polymers are colored due to the incorporation of the CTA.
The synthesis of polystyrenes by RAFT polymerization has been reported. (See, Yang, Z., et al., Macromolecules, 36, 7446-7452, 2003, hereinafter, Yang, et al.). In this work, benzyl-4-vinyldithiobenzoate was used as a chain transfer monomer (CTM). The authors report the formation of polystyrene of branched structure with the dithioester bond located at every branching point. The branches were cleaved by an aminolysis reaction for branching analysis, yielding nearly monodisperse linear polystyrene chains. This suggests that, rather than random branching, the polymerization resulted in polystyrene stars with arms emanating from poly(benzyl-4-vinyldithiobenzoate) cores. Moreover, the placement of the dithioester bond at the branch points can be expected to impart an undesirable color and odor to the polymer.
There remains a need for a method for synthesis of randomly branched polymers, suited for bulk industrial manufacturing, which can have high molecular weight and broad molecular weight distribution.